The contents of Application No. Heisei 9-63770, with a filing date of Mar. 3, 1997 in Japan, are herein incorporated by reference.
a) Field of the Invention
The present invention relates to a solid state image sensor using a plurality of light receiving elements such as photodiodes.
b) Description of the Related Art
A previously proposed image sensor will be described below.
The previously proposed image sensor includes: a horizontal address scanning circuit; a vertical address scanning circuit; and a plurality of pixels (picture elements) each having a photodiode and a switching element such as a first MOS (Metal Oxide Semiconductor) transistor, these pixels being arranged in a two-dimensional form.
A structure of a representative one of the pixels will be described below.
The photodiode has a cathode connected to a fixed potential line such as a plus bias voltage line and has an anode connected to a first electrode such as a drain of the first MOS transistor. A drive electrode such as gate of the first MOS transistor is connected to a corresponding one of the vertical address lines extended from the vertical address scanning circuit. A second electrode such as a source of the first MOS transistor is connected to a corresponding one of data lines. Each of the data lines is connected to an output line via another switching element of a second MOS transistor. The output line is connected to an output buffer. An output end of the buffer is connected to an output terminal of the image sensor.
A gate of the first MOS transistor connected to the data line is connected to the corresponding one of the horizontal address lines. The vertical address scanning circuit and the horizontal address scanning circuit are constituted by shift registers, respectively.
In the previously proposed image sensor, an optical-to-electric converted charge developed according to an incident light quantity on each of the photodiodes is stored in each corresponding one of the photodiodes during a turn-off state of the corresponding first MOS transistor in each corresponding one of the pixels.
In this state, at first, when the corresponding vertical address line is selected (becomes active), the first MOS transistor in each corresponding one of the pixels is, in turn, turned on so that the charge stored in the photodiode of the corresponding one of the pixels is distributed into the corresponding one of the data lines. Next, when the horizontal address line connected to the corresponding data line is selected, the MOS transistor is turned on so that the data line is connected to the output line and the charge in the same data line is supplied to the output buffer via the output line. Consequently, the charge stored in the photodiode can be read.
Such a previously proposed image sensor as described above is exemplified in FIG. 5.34 in page 108 of a Japanese Technical Book titled xe2x80x9cImage Input Technical Handbook xe2x80x9d authored by Yuji Kiuchi and published on Mar. 31, 1992.
However, false signals such as a smear and a shading are developed in the previously proposed image sensor.
It is noted that the subsequent explanation on the previously proposed image sensor will be advanced with the pixels of the image sensor constituted by four of a first pixel, a second pixel connected to the same data line as the first pixel, a third pixel connected to the same vertical address line as the first pixel, and a fourth pixel connected to the same data line as the third pixel, the data lines constituted by first and second data lines, and the first data line being common to the first and second pixels and the second data line being common to the third and fourth pixels.
First, a problem of the smear will be described.
While any one of the vertical address lines is selected, all photodiodes in a common row of the pixels connected to the corresponding one of the vertical address lines, e.g., the first and third pixels or the second and fourth pixels, connected to the selected vertical address line via the gates of the first MOS transistors in the common row of the pixels are connected to the respectively corresponding data lines, e.g., the first and second data lines.
For example, suppose that, in order to read the charge in the photodiode of the first pixel, the first vertical address line is selected. At this time, at the same time when the charge stored in the photodiode of the first pixel is distributed into the first data line, the charge stored in the photodiode of the horizontally adjacent third pixel is also distributed into the second data line. On the other hand, since, at this time, the second MOS transistor connected to the second data line is in the turned off state, the second data line indicates a high impedance state.
Hence, the distributed charge from the third pixel is stored in the second data line so that a potential of the second data line is raised.
If the incident light quantity in the photodiode of the third pixel horizontally adjacent to the first pixel is increased, the charge quantity therein is accordingly increased so that the potential of the second data line becomes high. Consequently, the increased potential of the second data line exceeds a threshold value of the first MOS transistor of the fourth pixel which is connected to the same second data line.
Hence, since the first MOS transistor of the fourth pixel horizontally adjacent to the second pixel and vertically adjacent to the third pixel cannot be maintained at its turn-off (unconducted) state any more, the charge in the second data line leaks into the photodiode of the fourth pixel and is stored in the corresponding photodiode of the fourth pixel.
The above-described phenomenon is called a data line saturation and provides a cause of the smear.
Unless the data line saturation occurs, a clear image can be obtained. If certain data lines of any of the rows of the pixels concerned with a light source is saturated, such a smear as if an intense light were radiated into the pixels in the corresponding rows appears on the image.
The smear can occur regardless of whether each of the first and second MOS transistors is a p-type or n-type. In addition, the smear occurs even when the photodiodes are used and operated in a solar battery mode.
In order to prevent the smear from being developed in the image sensor, such a measure as a lowering of a dynamic range of each MOS transistor and/or the buffer would be considered. However, when a field photographing or a night photographing is carried out, a sufficiently clear image cannot be obtained due to its narrowed dynamic range.
Next, the problem of the shading developed in the previously proposed image sensor will be described.
Suppose a situation such that when the image sensor is used and operated under a black darkness condition, i.e., when no incident light is present for all photodiodes, the reading operation is carried out. At this time, the shading phenomenon often occurs. In the reading operation, the first vertical address line is, at first, selected. Then, the first horizontal address line, the second horizontal address line, and so forth are sequentially selected.
When the first MOS transistor, e.g., in the first pixel or the second MOS transistor, e.g., connected to the first data line is turned on, an external charge is supplied to the corresponding MOS transistor and part of the external supplied charge leaks onto a semiconductor substrate via a parasitic capacitance formed between its gate and the substrate. The leaked charge onto the substrate is injected due to a capacitance coupling and diffusion into the source and drain of the MOS transistors of the third and fourth pixels.
At this time, since the second MOS transistor intervened in the second data line is in the turn-off state, the second data line indicates the high impedance state and the leaked charge is stored on the second data line connected to the source of the first MOS transistor of the fourth pixel.
Next, when the second horizontal address line is selected, the second MOS transistor connected to the second data line is turned on so that the stored and leaked charge is read as the false signal. As the scanning of the address lines is advanced, the leaked charges are accumulated. Consequently, the image such that a brightness is increased in the direction toward which the scanning is advanced appears.
Furthermore, the previously proposed image sensor cannot undergo a random access when the image sensor output signal is used for an image processing purpose.
That is to say, when the image processing is carried out, it is necessary to have a random access function such that an arbitrary pixel is read in an arbitrary sequence.
However, in the previously proposed image sensor, the reading operation is divided into two steps. First, one of the vertical address lines is selected so that the charges stored in the photodiodes of the corresponding pixels in the corresponding column are distributed into the correspondingly connected data lines. Next, one of the horizontal address lines is selected. Then, the charges in the respectively corresponding data lines are read from the output line.
For example, when the charge in the photodiode of the first pixel is read, the charge in the photodiode of the third pixel is also distributed into the second date line.
Next, when the charge in the photodiode of the fourth pixel is tried to be read, a dummy read is needed to be carried out to discard the charge in the second data line and to reset the second data line. When the charge in the photodiode of the fourth pixel is read, the charge in the photodiode of the third pixel is discarded. Consequently, at the subsequent stage, the charge in the photodiode of the third pixel cannot be read.
It is, therefore, an object of the present invention to provide an image sensor which can prevent. the false signals such as resulting in the smear and shading from being developed and whose optically-to-electrically converted charge in each pixel can be read in a random access manner.
The above-described object can be achieved by providing an image sensor comprising: a plurality of data lines; a plurality of pixels arranged in a two-dimensional form, each pixel to which both a horizontal address and a vertical address are allocated and including a light receiving element and a switching element for drivingly connecting the light receiving element to one of the data lines which is connected to the same pixel; a plurality of horizontal address lines, each horizontal address line being connected to the pixels to which the same horizontal address is allocated; a plurality of vertical address lines, each vertical address line being connected to the pixels to which the same vertical address is allocated, the switching element connecting the light receiving element in the same pixel to the one of the data lines which is connected to the same pixel when both of the horizontal address and the vertical address allocated to the same pixel are assigned through one of the horizontal address lines which is connected to the same pixel and one of the vertical address lines which is connected to the same pixel; an output line connected to the data lines and having one end serving as an output terminal; and a reading block interposed between each of the data lines and the output line and having a predetermined low input impedance with respect to a connection of an input end thereof to each of the data lines.
The above-described object can also be achieved by providing an image sensor comprising: a plurality of data lines; a plurality of pixels arranged in a two-dimensional form, each pixel to which both of a horizontal address and a vertical address are allocated and including optical-and-electrical converting means for converting an incident light quantity into a corresponding electrical charge and switching means for drivingly connecting the optical-to-electrical converting means to one of the data lines which is connected to the same pixel; a plurality of horizontal address lines, each horizontal address line being connected to the pixels to which the same horizontal address is allocated; a plurality of vertical address lines, each vertical address line being connected to the pixels to which the same vertical address is allocated, the switching means connecting the optical-to-electrical converting means in the same pixel to the one of the data lines which is connected to the same pixel when both of the horizontal address and the vertical address allocated to the same pixel are assigned through one of the horizontal address lines which is connected to the same pixel and one of the vertical address lines which is connected to the same pixel; an output line connected to the data lines and having one end serving as an output terminal; and reading means interposed between each of the data lines and the output line and having a predetermined low input impedance with respect to a connection of an input end thereof to each of the data lines.